The objectives are three fold: (1) to determine if interruption of pathways from retinal photoreceptors to the mediobasal hypothalamus (MBH) will prevent circannual (photoperiod-induced) and diurnal changes in patterns of tonic secretion of luteinizing hormone (LH) and prolactin in the ewe, (2) to define the neural or neuro-humoral pathways by which photoperiod influences the functions of the MBH and anterior pituitary, and (3) to compare the neural substrate which regulates the circadian and preovulatory surge nodes of estrogen-induced LH release in the ewe. Both ovariectomized ewes treated with estradiol and intact ewes will be subjected either to natural photoperiod out of doors or to artificially regulated photoperiods of 8L:16D or 16D:8L in photochambers. The effect of hypothalamic deafferentation, pinealectomy and discrete hypothalamic lesions on the resulting changes in the secretory patterns of LH and prolactin will be monitored by radioimmunoassay. The location of the lesions will be verified by histological procedures.